Observation of pn-junction depth in Mg 2 Si diodes fabricated by short period thermal annealing

We have fabricated Mg2Si pn-junction diodes by short-period (30 sec and 1 min) annealing between 400 °C and 480 °C and evaluated the pn-junction depth by EBIC and VC observations. The pn-junction depth of approximately 6 and 30 μm was formed for the annealing conditions of 400 °C, 30sec and 480 °C, 1min, respectively. By comparing the Ag depth profiles determined by SIMS, we estimated the activation ratio at the pn-junction of approximately 8 and 10 % for annealing at 450 °C and 480 °C, respectively. All diodes showed clear rectifying behavior in J-V characteristic and the spectral photosensitivity below approximately 1.8 μm under zero bias condition.


Introduction
Infrared photodetector is one of the key devices for applications in the fields of night vision, tele-communications, thermophotovoltaics and medical imaging [1][2][3][4][5].Magnesium half silicide (Mg2Si) is an indirect gap type semiconductor with the band gap energy of about 0.6 eV at room temperature [6][7][8].The band gap energy is able to control as far as 0.3 eV by making alloy compound with Mg2Si and Mg2Sn [9][10][11].Therefore, Mg2Si and its alloy compounds are expected to be utilized for the infrared photodetector with the cut-off wavelength between 2 and 4 µm [7,12].
Previously, we reported the fabrication of Mg2Si pn-junction diode by diffusing Ag dopant into n-type Mg2Si substrate and its photoresponse below 2 µm [7].The photosensitivity of the diode is improved by making the shallower pn-junction and the ring electrode in place of the opaque circular electrode [12,13].However, those reported diodes had deep pn-junction more than 75 µm, because of the high thermal diffusion temperature (550 ℃) and the relatively long annealing time (4 min) compared with the high diffusivity of Ag in Mg2Si [11].In order to further improve the photosensitivity of the diode, controlling the junction depth and forming the shallower junction are important, especially.
In this study, we report the fabrication of Mg2Si pn-junction photodiodes by short-period (30 sec and 1 min) annealing between 400 ℃ and 480 ℃ and the observation of their junction-depth depended on the annealing conditions.

Experimental Procedures
Fig. 1 shows the schematic diagram of the fabrication process of Mg2Si pn-junction diodes.The substrates of n-type Mg2Si with the size of about 3 × 3 × 0.8 mm and the electron density n = 6 × 10 15 cm -3 were prepared from a Mg2Si bulk single crystal grown by the modified vertical Bridgeman method [7,8,14].The surface of the substrates was lapped by SiC abrasive (MARUTO Co. Ltd. #8000) and polished subsequently like a mirror-face by fumed silica (AKASEL wafer free 0.2 µm).
The pn-junction was made by the thermal diffusion of Ag accepter.Diffusion source of Ag layer was evaporated onto the polished surface through a metal mask of 0.6 mm-diameter holes using a conventional resistive heating evaporator.After the deposition of Ag layer, Au metal was evaporated on the Ag to prevent an oxidation of Ag layer and form an ohmic electrode.The thickness of Ag layer and Au layer was about 400 nm and about 50nm, respectively.The thermal diffusion of Ag was carried out between 400 ℃ and 480 ℃ for 30 sec and 1 min in an Ar gas flow atmosphere by using an infrared rapid heating furnace (Mila 3000, ULVAC Co. Ltd.).
The pn-junction depth of Mg2Si photodiode was observed by the electron beam induced current (EBIC) under the zero bias condition and the voltage contrast (VC) techniques at room temperature using HITACHI SU8230.The acceleration voltages of an electron beam in EBIC and VC observations were 30 kV and 1 kV, respectively.The Ag diffusion profiles were evaluated by secondary ion mass spectroscopy (SIMS) on the 3°-off angle polished surface.The current density voltage (J-V) characteristics were measured by DC current source meter (KEITHEY 2400) under a dark condition.The photoresponsivity of the fabricated diodes were measured by a conventional lock-in technique using a halogen lamp chopped and passed through an IR filter (cut-off wavelength =1.2 µm) and a monochromator (JASCO CO. Ltd., CT-50) with a focal length of 500 mm at room temperature.The output power of infrared light was calibrated with a commercial PbS detector.

Result and Discussion
Figs. 2(a) and 2(b) present the top-views of Mg2Si substrates before and after annealing.The annealing temperature and period were 480 °C and 1 min, respectively.There is no significant change in both morphology and shape of Au electrode before and after annealing as similar to our previous reports [12].

Table I.
The rectification ratio at ±1 V and J Dark at -1 V of the fabricated diodes annealed between 400 ℃ and 480 ℃.

011101-3 JJAP Conf. Proc. , 011101 (2017)
Figs. 4(a) and 4(b) show typical cross-sectional EBIC images near the pn-junction of the diodes fabricated under the diffusion temperature at 450 ℃ for 30 sec and 1 min, respectively.Clear EBIC contrast showing the pn-junction interface varied depending on the diffusion periods and temperatures.The average junction depth determined from the EBIC and VC observations are listed in Table Ⅱ.The pn-junction depth below approximately 30 µm was formed by the short period thermal annealing between 400 ℃ and 480 ℃.Fig. 5 shows the plots of Ag concentration determined by SIMS analysis and the Ag-diffusion profiles calculated from the two stream diffusion equations [15,16] as follows: where N is Ag concentration, N1 eq and N2 eq are Ag concentration at equilibrium, W is diffusion depth, D1 and D2 are diffusion coefficient, t is annealing period.The parameters used for the diffusion profiles were determined from the detailed study of Ag diffusion in Mg2Si [15] and listed in Table Ⅲ.N1 eq and N2 eq indicate the Ag-substitutional diffusion and Ag-interstitial diffusion.The details about the Ag diffusion mechanisms in Mg2Si will be reported elsewhere in the near future.The data of measured Ag concentration agree well with the calculated diffusion profiles in both at 450 ℃ and 480 ℃.Since the charge density in p-and n-region is balanced at the interface of pn-junction, comparison of the results in Table Ⅱ and Fig. 5 may give the activation ratio of Ag in the diffused region.Estimated Ag concentration and Ag activation ratio at the interface of comparison are listed in Table Ⅳ.The activation ratios of Ag dopant are approximately 8 and 10 % for the diodes annealed at 450 ℃ and 480 ℃, respectively.These values are similar to our previous work [15].
The all fabricated diodes showed clear photoresponse under zero bias condition for the light incidence from the front side (Au-electrode side), although the Au electrode is non-transparent.This observation suggests that generated photo-carriers around the electrode would diffuse to the junction region and act as photo current.Fig. 6 represents spectral photosensitivities observed from the Mg2Si diodes fabricated by short period annealing at 450 ℃.Photoresponse below approximately 1.8 µm in wavelength was observed in both diodes, and their sensitivities increased monotonically with decreasing the wavelength.The cut-off wavelength is approximately 2.0 µm (0.61 eV) for Mg2Si photodiodes [7,8,12], but our experimental wavelength threshold of photosensitivities was approximately 1.8 µm.This would be due to the weak photo sensitivity in the wavelength range of 1.8-2.0µm.

Conclusion
We have fabricated Mg2Si pn-junction diodes by short-period (30 sec and 1 min) annealing between 400 ℃ and 480 ℃ and measured the pn-junction depth by EBIC and VC observation.All diodes showed clear rectifying behavior in J-V measurement, confirming the formation of pn-junction.The pn-junction depth approximately 6 and 30 µm was obtained for the annealing conditions of (400 ℃, 30 sec) and (480 ℃, 1 min), respectively.We also estimated the activation ratio at the pn-junction by comparing the Ag depth profiles with the junction depth.The values were approximately 8 and 10 % for annealing at 450 ℃ and 480 ℃, respectively.We found the photoresponse below about 1.8 µm from the diodes.

Fig. 5 .
Fig. 5. Ag concentration determined by SIMS and the calculated Ag-diffusion profiles (broken lines) of the diodes annealed at 450 ℃ and 480 ℃.

Fig. 6 .
Fig. 6.Spectral photosensitivity of Mg2Si pn-junction diodes fabricated by thermal annealing at 450 ℃ for 30 sec and 1 min.The measurement was carried out under zero bias condition at room temperature.

Table Ⅱ .
Diffusion conditions and pn-junction depths determined from EBIC and VC observations.

Table Ⅳ .
Estimated Ag concentration and activation ratio of Ag dopant at the pn-junction interface.